Various formation evaluation (FE) tools are used in hydrocarbon exploration and production to measure properties of geologic formations during or shortly after the excavation of a borehole. The properties are measured by formation evaluation tools and other suitable devices, which are typically integrated into a bottom hole assembly (BHA).
Such tools provide for making downhole measurements versus “measured depth” (which is the distance to the surface along the wellbore path) and/or versus time for one or more physical quantities in or around a borehole. The taking of these measurements may be referred to as “logging”, and a record of such measurements may be referred to as a “log”. Many types of measurements are made to obtain information about the geologic formations. Some examples of the measurements include gamma ray logs, nuclear magnetic resonance logs, neutron logs, resistivity logs, and sonic or acoustic logs or, for station logs, formation fluid pressures. True vertical depth (TVD) is the vertical distance between a downhole location and the surface. Various downhole physical properties depend on TVD but not on measured depth. For example, over a region of fluid filled permeable rock, the change in fluid pressure, ΔP, equals the product of the fluid density, ρ, with the acceleration of gravity, g, and change in true vertical depth, ΔTVD (i.e., ΔP=ρgΔTVD). Similarly, overburden pressure depends on TVD and, in turn, sound speed through rock depends on overburden and other factors.
Examples of logging processes include measurement-while-drilling (MWD) and logging-while-drilling (LWD) processes, during which measurements of properties of the formations and/or the borehole are taken downhole during or shortly after drilling. The data retrieved during these processes may be transmitted to the surface, and may also be stored with the downhole tool for later retrieval. Other examples include logging measurements after drilling, wireline logging, and drop shot logging.
FE tools often use real-time clocks that, when post-processing the logged data, allow the data to be correlated with associated times and depths. Such clocks allow individual measurements performed during logging to be assigned specific measured depths. One pre-condition for assuring accurate time (and thus measured depth) assignments is that both downhole and uphole clocks run synchronized.
The orientation of the logging tool is typically with respect to a vertical axis and magnetic north. Even small errors in determination of the borehole depth can corrupt logging data. An assumption that the logging instrument is moving smoothly through the borehole is not always valid due to rugose and sticky borehole conditions. Additionally, tool centralizers and decentralizers may keep the logging tool from moving smoothly. Horizontal deviations of the borehole may also lead to errors in measuring the borehole depth. It is, therefore, important to know the “true vertical depth” of the logging instrument as well as knowing the measured depth along the wellbore.